Angular is Vulnerable to XSRF Token Leakage via Protocol-Relative URLs in Angular HTTP Client

The vulnerability is a Credential Leak by App Logic that leads to the unauthorized disclosure of the Cross-Site Request Forgery (XSRF) token to an attacker-controlled domain.

Angular's HttpClient has a built-in XSRF protection mechanism that works by checking if a request URL starts with a protocol (http:// or https://) to determine if it is cross-origin. If the URL starts with protocol-relative URL (//), it is incorrectly treated as a same-origin request, and the XSRF token is automatically added to the X-XSRF-TOKEN header.

Impact

The token leakage completely bypasses Angular's built-in CSRF protection, allowing an attacker to capture the user's valid XSRF token. Once the token is obtained, the attacker can perform arbitrary Cross-Site Request Forgery (CSRF) attacks against the victim user's session.

Attack Preconditions

1. The victim's Angular application must have XSRF protection enabled.
2. The attacker must be able to make the application send a state-changing HTTP request (e.g., POST) to a protocol-relative URL (e.g., //attacker.com) that they control.

Patches

• 19.2.16
• 20.3.14
• 21.0.1

Workarounds

Developers should avoid using protocol-relative URLs (URLs starting with //) in HttpClient requests. All backend communication URLs should be hardcoded as relative paths (starting with a single /) or fully qualified, trusted absolute URLs.